I'd like to get some opinions from the members concerning the use of non-Lego pieces in a custom MOC. While I strive to keep traditional non-Lego pieces out of my bin (Mega-Bloks for example), my question is geared more towards the use plastic pieces to accentuate structures.[url][/url]Since my MOC is Blacktron I themed, it isn't in my budget to fork over $20 dollars or more for the rare trans-yellow quarter panels offered on Bricklink. Nor did Lego create a 1x6x5 window frame in black (only blue, brown, and grey) My idea was to use transparent yellow pieces of acrylic (Plexiglass) for use as a large panoramic window for one of my buildings.

Is this completely frowned upon? Or is it acceptable to a degree considering that the piece needed wasn't created by Lego?

One piece of advice before I contradict myself: Build for yourself, not for others. Build what you want, the way you want, and you'll be more satisfied than if you rely on the feedback of others.

That said:-There are builders who modify Lego elements to fit their concepts; dyeing/painting official elements that never appeared in certain colors is pretty forgivable. Cutting/bending/otherwise warping elements is taken well when it achieves something interesting; it tends to be less widely accepted than painting.-There are builders who included non-Lego elements in their builds. This tends to be taken poorly by Lego purists.-Certain themes (especially Technic and robotics applications, as well as train and certain factions of military builders) frequently use aftermarket elements to enhance their builds, often filling in gaps that Lego doesn't or won't produce. This is usually accepted but there are exceptions.-Contests have their own rules for what's allowed- follow them if that's what you're building for.

Personally, I wouldn't take issue with trans-yellow acrylic used for large viewports (if it matched the official Lego color).